System and method for reducing NOx emissions during transient conditions in a diesel fueled vehicle with EGR

ABSTRACT

The present invention is a dual-stage fuel injection strategy for compression ignition engines in which 15-40% of the fuel is injected into the combustion chamber no later than about −20 to −30 CA ATDC and as early as IVC. The remaining fuel is then injected in one or more fuel pulses, none of which start before about −20 to −30 CA ATDC. The fuel injected early in the compression stroke forms a lean mixture that burns with low soot and low NOx emissions. The combustion of that fuel serves to increase in-cylinder temperature such that the ignition delay of subsequent fuel injection pulses is short. This mode is utilized when it is predicted that a NOx spike is imminent. Various other alternative methods for reducing NOx spikes are also disclosed such as specialized EGR systems that can provide EGR with low manifold vacuum.

This application is related to Ser. No. ______ Docket Number 202-0338,titled, “SYSTEM AND METHOD FOR REDUCING NOx EMISSIONS DURING TRANSIENTCONDITIONS IN A DIESEL FUELED VEHICLE”, assigned to the assignee of thepresent application, and filed on the same day as the presentapplication. The entire contents of 202-0338 are hereby incorporated byreference into the present application.

FIELD OF THE INVENTION

The present invention relates generally to the control of an internalcombustion engine powered by diesel fuel, and more specifically toreducing transient NOx generation produced by such a vehicle.

BACKGROUND OF THE INVENTION

Controlling NOx emissions in diesel engines has posed significantchallenges to the automotive industry. While emission control devices,such as NOx catalysts, can be used, these devices may be insufficient tomeet ever-increasing emission standards.

A method to reduce NOx in diesel engines is the use of exhaust gasrecirculation (EGR). EGR reduces NOx emissions during steady, or nearsteady, engine operation.

However, under transient engine operation in which a vehicle is requiredto accelerate, EGR can limit the performance of the vehicle by reducingthe amount of airflow through the engine. EGR reduces airflow bydisplacing air in the combustion chamber, heating up the intake charge,and redirecting exhaust gas that would normally go through theturbocharger to the intake manifold. This last effect reduces the energyflow through the turbine, thus restricting the engine's ability tocreate boost. This phenomenon can be dubbed “the EGR-Boost” tradeoff.

Typically, conventional diesel systems suspend the use of EGR in orderto accelerate aggressively. However, the inventors herein haverecognized that without EGR, NOx emissions (concentration) increasedramatically. This comes at a time when the air mass flow rates are veryhigh, causing NOx production to spike. Thus, the place where EGR is mostneeded is the place where it is not used.

The difficulty of this problem can be further appreciated by consideringtwo types of EGR system that could be used with turbo-charged engines:the high pressure system and the low pressure system. The inventorsherein have recognized the following disadvantages with each system.

In heavy duty applications, high pressure EGR systems have difficultyproviding sufficient EGR flow at high load conditions, especially at lowspeed, since there is a negative pressure differential between theexhaust and intake manifold. One method to improve EGR flow is tothrottle the engine. However, throttling the engine increases enginepumping work and decreases the gas flow to the engine.

Similarly, low pressure EGR systems allow only limited amounts of EGRunder light load conditions, especially at low speed, since there islittle pressure differential. Further, the low pressure EGR system addssignificant purging volume that causes delays when trying to accelerate.

SUMMARY OF THE INVENTION

The above disadvantages are overcome by a system for an engine having anintake and exhaust manifold, the engine having a compression devicecoupled with a first portion coupled to the intake and a second portioncoupled to the exhaust manifold of the engine, the system comprising: afirst exhaust gas recirculation system having a first end coupled to theexhaust manifold upstream of the second portion of the compressiondevice and a second end coupled to the intake manifold downstream of thefirst portion compression device, said first system also having a firstvalve that adjusts a first flow amount from the exhaust manifold to theintake manifold; a second exhaust gas recirculation system having afirst end coupled downstream of the second portion of the compressiondevice and a second end coupled to the intake manifold, said firstsystem also having a second valve that adjusts a second flow amount fromthe exhaust to the intake manifold.

By providing multiple EGR loops, it is possible to reduce NOx emissionsduring high engine load, even in the presence of a compression devicesuch as a supercharger. In other words, by using both a two EGR loops,one can obtain the benefits of each of the high pressure and lowpressure EGR systems and thereby minimize the disadvantages of eachsystem since the two systems complement each other.

Another advantage of the present invention is the ability to reducetransient NOx spikes.

Yet another advantage of the present invention is the ability ofmultiple loop EGR systems to enable the use of EGR throughout the entireengine map without using a throttle.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages described herein will be more fully understood by readingan example of an embodiment in which the invention is used to advantage,referred to herein as the Description of the Preferred Embodiment, withreference to the drawings wherein:

FIG. 1 is a graph showing NOx production of a vehicle during a drivecycle;

FIG. 2 is a block diagram of an engine in which the invention is used toadvantage;

FIGS. 3 and 5 are flowcharts illustrating control methods of the presentinvention;

FIG. 4 illustrates engine maps utilized in the present invention;

FIG. 6A shows intake manifold pressure required to maintain a 17:1 AFRgiven an amount of fuel;

FIG. 6B shows a naturally aspirated, unthrottled AFR given a desiredfuel amount (MFDES);

FIGS. 7A-D show detailed engine and vehicle test data that shows how EGRShuts off once MFDES exceeds 50 mg/stk, and how a negative timederivative of pressure marks the end of a NOx spike;

FIG. 8 shows example results utilizing the routines of FIGS. 3-5;

FIG. 9 shows a schematic of one proposed strategy, which involvesinjecting a significant portion of the fuel into the combustion chamberearly during the compression stroke;

FIG. 10 shows predicted NOx emissions results from a simulation of theproposed strategies in Table 1;

FIGS. 11 and 12 show detailed simulation data of the engine combustion;

FIG. 13 shows predicted heat release from a simulation of the presentinvention;

FIG. 14 shows the rate of Pressure Rise for the simulation data;

FIG. 15 illustrates various configurations of the present invention; and

FIGS. 16 a-16 f show alternative embodiments of the present inventionwith respect to the block diagram of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows NOx emissions during a transient emissions cycle of avehicle powered by a typical turbocharged diesel engine. The Figuredemonstrates that spikes in NOx emissions occur at various times duringtransient operation. Those spikes typically occur during hardacceleration and the primary reason for their existence is theinteraction between the turbocharging system and the typical highpressure EGR system. During hard acceleration, the use of EGR issuspended in order to both divert exhaust flow through the turbine,which allows the turbocharging system to create boost, and increases airflow through the engine. However, without EGR, NOx emissions(concentration) increase dramatically. This comes at a time when the airand thus exhaust mass flow rate are very high causing NOx production tospike dramatically.

During the a typical urban driving cycle, the time in which the engineis operated under conditions that produce these spikes accounts for onlyabout 4-5% of the total cycle time. However, approximately 30-45% of theNOx produced during the cycle comes from these NOx spikes, asillustrated in FIG. 1.

The present invention provides several methods to overcome these NOxspikes.

FIG. 2 shows an example of an internal combustions engine system.Specifically, internal combustion engine 10, comprising a plurality ofcylinders, one cylinder of which is shown in FIG. 2, is controlled byelectronic engine controller 12. Engine 10 includes combustion chamber30 and cylinder walls 32 with piston 36 positioned therein and connectedto crankshaft 40. Combustion chamber 30 communicates with intakemanifold 44 and exhaust manifold 48 via respective intake valve 52 andexhaust valve 54.

Exhaust air/fuel ratio sensors can also be used in the presentinvention. For example, either a 2-state EGO sensor or a linear UEGOsensor can be used. Either of these can be placed in the exhaustmanifold 48, or downstream of devices 19 a, 22, or 20.

Intake manifold 44 communicates with throttle body 64 via throttle plate66. In one embodiment, an electronically controlled throttle can beused. In this case, the throttle can be used to throttle airflow to helpdrive in more EGR. In one embodiment, the throttle is electronicallycontrolled to periodically, or continuously, maintain a specified vacuumlevel in manifold 44. Intake manifold 44 is also shown having fuelinjector 68 coupled thereto for delivering fuel in proportion to thepulse width of signal (fpw) from controller 12. This configuration isone potential way to get a pre-mixed mixture for dual-stage combustion,as is known in the art. Fuel is delivered to fuel injector 68 by aconventional fuel system (not shown) including a fuel tank, fuel pump,and fuel rail (not shown). In the case of direct injection engines, asshown in FIG. 2, a high pressure fuel system is used such as a commonrail system. However, there are several other fuel systems that could beused as well such as EUI, HEUI, etc. In the embodiment described herein,controller 12 is a conventional microcomputer, including: microprocessorunit 102, input/output ports 104, electronic memory chip 106, which isan electronically programmable memory in this particular example, randomaccess memory 108, and a conventional data bus.

Controller 12 receives various signals from sensors coupled to engine10, in addition to those signals previously discussed, including:measurements of inducted mass air flow (MAF) from mass air flow sensor110 coupled to the air filter [A on FIG. 2] (note that in a dieselengine the air flow meter is typically read before the compressor, alsonote that the airflow sensor should be placed before the entrance pointfor the low pressure EGR loop); engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling jacket 114; a measurement ofmanifold pressure (MAP) from manifold pressure sensor 205 coupled tointake manifold 44; a measurement of throttle position (TP) fromthrottle position sensor 117 coupled to throttle plate 66; and a profileignition pickup signal (PIP) from Hall effect sensor 118 coupled tocrankshaft 40 indicating and engine speed (N). (Further, controller 12receives a measurement of manifold temperature (Te) from sensor 76.Alternatively, sensor 76 can provide an indication of exhaust gastemperature, or catalyst temperature.) In an alternative embodiment, thetemperature measured is typically into and/or out of catalyst(s) and/orparticulate filters, rather than in the exhaust manifold, since therewill be substantial temperature drop in the turbine.

Exhaust gas is delivered to intake manifold 44 by EGR tube 202communicating with exhaust manifold 48. EGR valve assembly 200 islocated in EGR tube 202. Stated another way, exhaust gas travels fromexhaust manifold 44 first through valve assembly 200, then to intakemanifold 44. EGR valve assembly 200 can then be said to be locatedupstream the intake manifold. There is also optionally an EGR cooler [Yin FIG. 2] placed in EGR tube 202 to cool EGR before entering the intakemanifold. Cooling is typically done using engine water, but andair-to-air heat exchanged could also be used.

Pressure sensor 205 provides a measurement of manifold pressure (MAP) tocontroller 12. EGR valve assembly 200 has a valve position (not shown)for controlling a variable area restriction in EGR tube 202, whichthereby controls EGR flow. EGR valve assembly 200 can either minimallyrestrict EGR flow through tube 202 or completely restrict EGR flowthrough tube 202. Vacuum regulator 224 is coupled to EGR valve assembly200. Vacuum regulator 224 receives actuation signal (226) fromcontroller 12 for controlling valve position of EGR valve assembly 200.In a preferred embodiment, EGR valve assembly 200 is a vacuum actuatedvalve. However, any type of flow control valve may be used such as, forexample, an electrical solenoid powered valve or a stepper motor poweredvalve.

Also, particulate filter 20 and lean NOx catalyst 22 are shown coupledin the exhaust path downstream of compression device 19. Compressiondevice 19 can be a turbocharger or any other such device. Device 19 hasa turbine 19 a coupled in the exhaust manifold 48 and a compressor 19 bcoupled in the intake manifold 44 via an intercooler [X in FIG. 2] whichis typically and air-to-air heat exchanger, but could be water cooled.Turbine 19 a is typically coupled to compressor 19 b via a drive shaft15. (This could-also be a sequential turbocharger arrangement, singleVGT, twin VGTs, or any other arrangement of turbochargers that could beused.)

Further, drive pedal 70 is shown, along with a driver's foot 72. Pedalposition (pp) sensor 74 measures angular position of the driver actuatedpedal.

Controls methods for transient NOx reduction are now described withreference to FIGS. 3-5. Generally, this method relates to anticipatingthe NOx spikes in order to initiate action (e.g., start using dual-stagecombustion, start using dual EGR loops, etc.). Further, the methodrelates to recognizing the end of NOx spikes in order to discontinuethis action.

Referring now to FIG. 3, a routine is described for controlling enginemode of operation in response to predicted NOx emissions from engine 10.In step 312, the routine determines a required fuel injection amount, orfuel demand (MFDES) based on a pedal position (pp), engine speed (N),and measured mass air flow (MAF). Note that this fuel demand can also bedetermined based on other parameters. For example, a two dimensional mapof engine speed and pedal position can be used. Alternatively, atwo-dimensional map of pedal position and vehicle speed can also beused. Moreover, the required fuel is based on a speed error during idlespeed control conditions such as below a predetermined vehicle speedwith pedal position below a specified value.

Next, in step 314, the routine calculates the manifold pressure (MAP)and the rate of change of manifold pressure (ΔMAP). In this particularembodiment, the change in manifold pressure is approximated as thedifference in manifold pressure between the current sample and theprevious sample. In one example, manifold pressure is measured fromsensor 205. Note that the desired EGR amount and the EGR valve positionfor the common high pressure loop will depend on the mode of operationchosen from the routine that determines whether a NOx spike ispredicted/detected/occurring.

Next, in step 320, the routine determines whether a NOx spike ispredicted/determined as described in more detail below with particularreference to FIG. 5. In one particular example the routine checks theflag (spike_flg). Note that the spike_flg should be initialized to zerowhen the engine starts up. Different actions should be taken given theprevious value of spike_flg as described herein with regard to FIG. 5.When the answer to step 320 is no, the routine continues to step 322 anduses normal engine operating modes. For example, EGR is delivered usingthe high pressure EGR loop and the desired fuel (MFDES) is injected asdone during steady-state operation.

When the answer to step 320 is yes, the routine continues to step 324where an alternate mode is performed to reduce NOx generation. The firststep to all of the alternative NOx reduction strategies is to shut offthe flow of EGR through the conventional high pressure EGR loop in orderto quickly increase engine boost by maximizing the exhaust flow throughthe turbine. In this example, a multiple injection strategy is used toprovide the desired fuel demand in such a way that a substantial portionof the fuel burns under fuel lean conditions, thus reducing NOxemissions. Alternatively, other multiple injection strategies could alsobe used to reduce NOx emissions. In another example, the high pressureEGR loop is shut off to maximize exhaust flow through the turbine and alow pressure EGR loop is used to deliver EGR to the intake air. Thus,according to the present invention, it is possible to reduce engine NOxgeneration while at the same time allowing engine boosting.

Referring now to FIG. 4, a two-dimensional map is shown where thedesired EGR amount (or MAF) is determined based on the fuel demand andengine speed. Optionally, this map can be extended to take into accountvarious engine-operating conditions such as engine coolant temperatureand manifold pressure. In this case, multiple maps can be used togenerate three-dimensional tables. In the particular example shown inFIG. 4, generally, as the required fuel and engine speed increase, thedesired MAF increases (or the desired EGR decreases). Such an approachis typically utilized with turbocharged diesel engines that use highpressure EGR systems because the EGR increases turbo lag and displacesair, thus hampering the engine performance.

However, according to the present invention, if multiple EGR loops areutilized, at least modest levels of EGR can be scheduled even at highfuel demands thereby allowing for decreased NOx emissions, as shown inFIG. 4A.

Referring now to FIG. 5, a routine is described for determining whenincreased NOx emissions, such as NOx spikes, occur. First, in step 500,the routine determines whether a NOx spike was in progress by examiningspike_flg.

If spike_flg=0 during step 500, then a NOx spike was not in progress andthe routine determines whether a NOx spike starting condition ispresent. Various parameters can be used to make this determination suchas, for example, whether the combustion air/fuel ratio is less thanabout 17:1, whether the desired fuel amount is greater than a fuelthreshold (MF threshold), and/or whether the EGR amount is lower than anEGR threshold. (One could also potentially use concentration of O2, NO,CO2, soot, or other relevant specie in the exhaust, engine torque, pedalposition, EGR valve position. However, the preferred method based on thedata analyzed is described below with reference to FIGS. 6 & 7. It isonly used to determine when the engine can no longer tolerate EGR orwhen the engine has decided to remove the EGR.) This is described inmore detail below with particular reference to FIGS. 6 and 7. When theanswer to step 510 is yes, the routine continues to step 512 and setsthe spike flag (spike_flg) equal to 1 and the routine ends. If theanswer to step 510 is no, the routine is terminated without changingspike_flg from its previous value of zero.

If spike_flg was equal to 1 during step 500, a NOx spike was already inprogress and, in step 514, the routine determines whether a NOx endingcondition is present. There are various methods for determining whetherthe NOx spike ending condition is present such as, for example, whethera change in manifold pressure is greater than a threshold, whether thischange in pressure is present for a predetermined time, whether desiredfuel amount is less than the threshold (including a small margin x),and/or whether engine load is decreasing. As described below, thisdetermination of a NOx spike is used to dynamically select betweenoperating modes in order to reduce tailpipe NOx emissions. Still otherconditions that can be used include a rate of change of desired torque,required fuel injection amounts, pedal position, air/fuel ratio(oxygen/fuel ratio), or species such as O₂, CO₂ in the intake orexhaust. When the answer to step 514 is yes, the routine continues tostep 516 to set the spike flag to zero and ends the routine. If theanswer to step 514 is no, the spike flag remains at 1 and the routineends.

Note that the methods described above allow for anticipation of a NOxspike, therefore giving a priori information that can be used to presetengine conditions to avoid the NOx spike generation.

As explained in FIG. 3, a spike_flg equal to 1 signals the engine toeliminate high pressure EGR to maximize exhaust flow through the turbineand operate in an alternate mode that is aimed at reducing NOx. Many ofthe methods used to reduce NOx emissions do so at the expense of sootand/or fuel economy. Consequently, it is typically not optimal tooperate the engine under those conditions when high pressure EGR can beused to reduce NOx instead. In order to optimize utilization of themethods outlined above (and below), a method should be found toanticipate NOx spikes so as to minimize the operation of the engineusing these methods. (Various ways to anticipate NOx spikes aresuggested above. The preferred method is below.)

As stated earlier, NOx spikes occur because EGR and boost appear to bemutually exclusive when a typical high pressure EGR loop is used. Themain reason for these NOx spikes during aggressive acceleration comesfrom the engine's need for air, which translates to a need for boost.When high pressure EGR is used, the flow of EGR robs the turbine used todrive the intake compressor of the energy it needs to create that boost.Consequently, high pressure EGR must be shut off during acceleration,which causes NOx emissions to increase dramatically. Thus, the onset ofNOx spikes is dictated by the point at which it is decided that theengine can no longer operate without boost. FIG. 6A shows the amount ofboost pressure required to maintain a 17:1 air/fuel ratio at variousvalues fuel flow rates (MFDES) when the engine is operated without EGR.MFDES is given in mg/stroke. The fuel flow rate that coincides withatmospheric pressure represents the maximum amount of fuel that can beburned at or above a 17:1 air/fuel ratio without boost. For this case,that value is 50 mg/stroke. To view this in a different way, FIG. 6Bshows the air/fuel ratio for a variety of fuel flow rates assuming noboost and no EGR. To reach air/fuel ratios higher than those on thecurve, boosting is required. The Figure shows that boosting is needed toattain at least a 17:1 air/fuel ratio when MFDES is 50 mg/stroke. Asstated earlier, in order to get boost, the level of EGR is usuallysignificantly reduced or shut off given the current EGR system. FIGS.7A-7D, and specifically FIGS. 7A-7B, show that the EGR valve does indeedclose once MFDES exceeds 50 mg/strk, and that this occurrence coincideswith the beginning of a NOx spike for the typical engine operation shownin FIG. 1. This is true over the entire cycle with very few exceptions.The data reveals that the closing of the EGR valve is not necessarily areliable harbinger to a NOx spike. However, MFDES exceeding a value thatcreates an AFR lower than 17:1 when the engine is naturally aspirated,appears to be an extremely reliable indication that a NOx spike isimminent.

Note that a 17:1 AFR without boost or EGR was used in this case as thepoint at which the engine could no longer be operated without boost.This was done because, typically at that point, diesel combustionefficiency drops of due to a lack of available oxygen. However, thechoice of air/fuel ratio used to decide this break point involves atradeoff between engine performance during acceleration and NOxemissions. The quicker high pressure EGR is shut off, the shorter theturbocharger lag is and the quicker the engine accelerates, but at theexpense of NOx emissions. The longer the EGR is used the lower the NOxis, but at the cost of longer turbocharger lag and poorer engineperformance. Further analysis shows that the end of a NOx spike ismarked by a drop in boost pressure, which is an indication that the loadhas decreased significantly.

FIGS. 7C-7D show a plot similar to that shown in FIGS. 7A-7B, but withthe boost pressure overlayed on the graph. The Figure shows that the endof the NOx spikes occurs 1 or 2 seconds after the time derivative of theboost pressure becomes negative.

One aspect of the present invention is based on using the informationdescribed above to predict the start and end of a NOx spike usingreadily available engine parameters. This information leads to thefollowing criterion for the particular engine selected:

-   -   A NOx spike begins when the fuel demand (MFDES) increases above        a value at which the air requirement of the engine exceeds what        can be achieved without boost (based on a minimum air/fuel ratio        of, for example, 17:1). This will signify the point where either        additional or alternative steps should be taken to enable        simultaneous NOx control and boosting.    -   When a NOx spike is in progress, the signal that the spike is        over should not be sought until the fuel demand drops below a        value that is equal to or somewhat less (2-3 mg less) than that        used to signify the start of the NOx spike. Using a value        somewhat less than that used in the previous step may help        prevent choppy engine operation caused by jumping in and out of        modes of operation too frequently.    -   A NOx spike ends when the manifold air pressure (boost) has        decreased over the last 1-2 seconds. This signifies a drop in        the airflow and engine load.

Note that this data is merely an example of the criteria that can beselected for use in the flow charts and routines described above hereinwith particular reference to FIGS. 3-5.

FIG. 8 shows the degree of success that can be achieved in determiningthe time and duration of NOx spikes using these guidelines. However, asdescribed above, various combinations of parameters can be used, withvarying accuracy.

The strategy described herein could facilitate drastic reductions in NOxduring transient cycles. The extent of the reduction would depend on thealternative mode of operation used to reduce NOx emissions without theuse of high pressure EGR. Some possible alternate modes of operationare:

-   -   1. To immediately remove most or all EGR and use dual-stage        combustion. Using this method and assuming that a 70% NOx        reduction would be obtained during that time (as projected from        combustion simulations), it is projected that a 33% reduction in        FTP cycle NOx emissions would result. However, it may be        possible to achieve even further reduction by combining the dual        stage combustion and corresponding control routines with EGR.    -   2. To remove EGR and use other split injection strategies aimed        at reducing NOx emissions. Split main injection has been shown        in the SAE literature (see SAE 960633).    -   3. Continue using EGR at high to moderate levels and use        Electric Assisted Boosting to increase intake pressure. No        projections of the NOx benefit for this method have been        developed. (Note, however, that this approach has a potential        drawback in that it will reduce and ultimately eliminate the EGR        driving force.)    -   4. Have both low pressure and high pressure EGR loops on the        engine. Close the high pressure EGR loop during NOx spikes and        use the low pressure EGR loop to provide a base level of EGR.        That level may be smaller than that used during steady state in        order to avoid displacing too much air. (Even a small level of        EGR would reduce NOx better during acceleration since the high        pressure EGR valve would have to be shut anyway.)

Next, a detailed explanation of the dual stage combustion is described.This is a method for diesel combustion in which combustion occurs in twoseparate stages, lean pre-mixed combustion and normal diesel combustion.Lean pre-mixed combustion is accomplished by introducing a significantportion of the fuel either into the combustion chamber very early in thecompression stroke through one or a series of pilot injections or intothe intake manifold during induction. The early introduction of thisfuel gives it enough time to mix with the air and form a lean (andpotentially homogeneous) mixture that ignites due to the increasedtemperature during compression, potentially in HCCI-like combustion. Theremainder of the fuel is injected in any number of injection events toproduce standard diesel combustion. This method of combustion obtainslow NOx emissions with very little smoke penalty.

Because its formation rates increase with temperature, NOx emissions areprimarily produced in an internal combustion engine in combustionregions where the local equivalence ratio is close to stoichiometric.Soot is formed in high temperature rich regions of the combustionchamber. Diesel combustion is a process in which fuel progresses throughboth of these regions, despite the fact that the overall equivalenceratio is usually fairly lean. Most of the fuel injected in a dieselengine is initially broken down under locally rich equivalence ratioseither in the premixed burn stage or in the fuel-rich premixed flame.These regions mainly produce CO, UHC, and soot precursors. That brokendown fuel ultimately proceeds through a thin diffusion flame that existsat or near stoichiometric equivalence ratios, where complete productsare produced and full heat release is achieved and NOx is produced (seeSAE 970873).

The present invention includes a combustion strategy designed to burnsignificant portion of the fuel under lean conditions, thus avoidingboth NOx and soot production.

FIG. 9 shows a schematic of the proposed strategy, which involvesinjecting a significant portion of the fuel into the combustion chamberearly during the compression stroke (first pulse), thus enabling thatfuel to mix with air and burn under lean air/fuel equivalence ratios.The remaining fuel is injected near top dead center (TDC) and burnednormally (second pulse). The fuel that is injected during the firstpulse is burned under lean conditions and produces an insignificantamount of NOx and soot emissions. The heat release from that fuel servesto heat the contents of the combustion chamber. This shortens theignition delay of the second pulse, thus reducing both NOx emissions andcombustion noise.

This concept was tested using simulations of the closed cycle portion ofa diesel engine (IVC to EVO). Simulations were conducted for a singleengine operating condition: 1500 rpm, 5 bar BMEP (˜30 mg/stroke).Thirteen different injection schemes were assessed. The baseline casewas a single injection pulse delivering 100% of the fuel starting at+3.2 CA ATDC. The basic injection parameters defining this baseline caseand the other twelve cases are shown in Table 1. TABLE 1 SimulationInjection Strategies Case 1st Inj. Qty. Dwell SOI Main Dwell Number (%of total) (CA) (CA ATDC) (CA) 1 0 N/A +3.2 N/A 2 7 10 +3.2 10 3 7 20+3.2 20 4 7 40 +3.2 40 5 7 90 +3.2 90 6 21 10 +3.2 10 7 21 20 +3.2 20 821 40 +3.2 40 9 21 90 +3.2 90 10 49 10 +3.2 10 11 49 20 +3.2 20 12 49 40+3.2 40 13 49 90 +3.2 90

FIG. 10 shows the NOx emissions results (simulated NOx emissions withpre-injection) for this study. The Figure shows that for all cases,except that of relatively close-coupled, large-quantity pilot injection,a substantial NOx reduction (50-70%) was achieved using early fuelinjection.

The simulations suggest that two major mechanisms contribute to thedecrease in NOx emissions using pre-injection. First, the fuel that ispre-injected has time to mix with air and create a lean mixture beforecombustion. This can be seen in FIG. 11, which shows a snapshot of thein-cylinder equivalence ratio just prior to main fuel injection in which21% of the fuel is pre-injected about 90 CA before TDC. This leanmixture burns at very low temperatures (see FIG. 12), thus producing lowNOx; it also produces low soot emissions because of the abundance ofoxygen. I.e., FIG. 11 shows the lean combustion of pre-injected fuel,and FIG. 12 shows the low temperature combustion of pre-injected fuel.

The second factor contributing to NOx reduction is a decrease in theignition delay caused by the addition of heat in the combustion chamber.The amount of fuel injected during ignition delay has a strongcorrelation with the amount of NOx produced by the engine. FIG. 13compares the heat release rate of the baseline (single injection) casewith that of two cases in which 21% of the fuel was pre-injected. TheFigure illustrates that early fuel injection significantly reducesignition delay. This results in a significant reduction in NOxemissions. The Figures also illustrates the ignition delay.

This method of NOx reduction has an added benefit to noise, much likepilot injection. FIG. 14 compares the rate of change of in-cylinderpressure, a quantity for which the maximum value is directly related tothe combustion noise, for three cases:

-   -   (1) Single injection.    -   (2) Close-coupled pre-injection of a small quantity of fuel.    -   (3) Pre-injection of a moderate quantity of fuel early during        compression stroke.

The Figure shows that noise is reduced significantly from a singleinjection case when part of the fuel is injected early in thecompression stroke. The Figure also shows that this noise reduction iscomparable to that of what should be representative of a conventionalpilot injection.

The following Figures and description relate to various types of EGRsystems that can be used to solve the EGR-boost problem described above.Some of these are alternative EGR systems in which all or part of theEGR flows from the exhaust line after a diesel particulate filter (lowpressure) to either a point before the compressor (low pressure) or tothe intake manifold (high pressure). Since the EGR gases flowing throughthese lines would still flow through the turbine, this EGR would notdiminish the performance of the turbine. Thus, the turbine would be ableto spin up quickly even with modest or possibly high levels of EGR. Forcases in which only some of the EGR flows through this low pressureloop, a second EGR loop, one that is identical to a conventional EGRloop, is used. In this case, only the low pressure loop is used whentrying to accelerate.

FIG. 15 shows a schematic view of engine 10, including compressor 19 b,turbine 19 a, intake manifold 44, exhaust manifold 48, particulatefilter 20, lean NOx catalyst 22, and intercooler 1500. For reference,points 1-7 are also labeled to facilitate the description below.

A typical EGR system takes exhaust gas from the exhaust manifold (5) andplumbs it back into the intake manifold (4). Such a system is shown inFIG. 2. For some cases, the flow rate of EGR is too high for the EGRplumbing capacity at natural pressure differences, thus requiring theuse of an intake throttle to depress the inlet air pressure and drivemore EGR. EGR that flows through this loop does not flow through theturbine, thus starving the turbine for input energy. The following showsthe benefits and consequences of using several alternate EGR systems.These systems were narrowed from a host of possibilities according tothe following criteria:

-   -   One should not pump dirty or sooty exhaust gas.    -   One should not put dirty or sooty exhaust gas into the        compressor.        Possible EGR Systems given the above criteria:    -   1. 5 to 4—Common High Pressure Loop (see FIG. 2)    -   2. 7 to 4—Low Pressure to High Pressure (see FIG. 16 a)    -   3. 7 to 1—Low Pressure Loop (see FIG. 16 b)    -   4. 5 to 4 & 7 to 4—dual-loop EGR system (see FIG. 16 c)    -   5. 5 to 4 & 7 to 1—dual-loop EGR system (see FIG. 16 d)    -   6. 7 to 2—Low Pressure to High Pressure using intercooler for        cooling EGR (see FIG. 16 e)    -   7. 5 to 4 & 7 to 2—dual-loop EGR system using intercooler for        cooling EGR (see FIG. 16 f)        Note that some of the equipment in the EGR loops described        herein is optional such as coolers, throttles, and pumps.        EGR System #1: 5 to 4 (see FIG. 2)

As stated, this is the basic EGR system, described in FIG. 2, which issuitable for use with the present invention, especially if the multipleinjection strategy described above is utilized. This system allows EGRflow without pumping (especially at light loads), and a lowfilling/purging volume (quick response). However, it also potentiallyhas a bulk flow pumping loss for throttling, dirty EGR (durability),less flow through the turbine-turbo lag, no EGR during transients-NOxcontrol problem, and coordinated control with EGR valve and intakethrottle.

EGR System #2: 7 to 4 (see FIG. 16 a)

In this system, EGR is brought from a point after the diesel particulatefilter to the intake manifold using an air pump (not shown). Anadditional cooler (not shown) is placed somewhere in the EGR loop inorder to cool the EGR before entering the combustion chamber. The flowrate of EGR is controlled using a valve. Such a system is also shown inFIG. 16 a. In particular, exhaust is routed from manifold 48 to theturbine 19 a. From there, it passes to particulate filter 20 and leanNOx catalyst 22. Exhaust gas downstream of device 22 is then routed topump Zb, and cooler Yb, before passing through the valve 200 b andentering manifold 44.

This system provides minimal dirty EGR (better durability), minimal bulkflow pumping losses (intake throttle can be eliminated if desired), allexhaust gas flows through the turbine (improved turbo response), lowfilling/purging volume (quick response), EGR can be used duringtransients-NOx control, and a simpler control system (if not using ITHto control EGR). However, all of the EGR is pumped (high pressure, highmass flow), all exhaust flows through DPF-increased back pressure, andthere is a longer EGR plumbing loop.

EGR System #3: 7 to 1 (see FIG. 16 b)

In this system, EGR is brought from a point after the diesel particulatefilter to a point before the compressor using an air pump or a venturi(not shown). A cooler (Ya) is also placed in the EGR loop in order tocool the EGR before entering the compressor. The flow rate of EGR iscontrolled using a valve such as valve 200. An optional pump Za is alsoshown. An optional intake throttle 117 is also shown.

Such a system gives minimal dirty EGR (better durability), minimal bulkflow pumping losses-(intake throttle can be eliminated), all exhaust gasflows through the turbine (improved turbo response), potential to use aventuri to pump EGR, EGR during transient-NOx control, and a potentiallysimpler control system (if throttle eliminated). Further, there ispotential for better EGR distribution/mixing. However, there may be aneed for a pump, all exhaust flows through DPF-increased back pressure,there is a longer EGR plumbing loop, and there is an increasedfilling/purging volume-degraded transient response.

EGR System #4: 5 to 4 & 7 to 4 (see FIG. 16 c)

In this system, shown in FIG. 2, two EGR loops are used. One EGR loopconnects the exhaust, manifold to the intake manifold. The other takesexhaust gases from a point after the DPF and pumps the into the intakemanifold using an air pump. When the vehicle is not accelerating, thefirst EGR loop is used to its fullest extent, and the second loop isused to supplement the EGR flow to avoid using the intake throttle. Whenthe vehicle is trying to accelerate (or when a NOx spike is predicted),the first EGR valve is shut to maximize the exhaust flow through theturbine, thus improving transient response of the engine. The secondloop is used to supply a base amount of EGR to control NOx emissions.Coolers (not shown) and pumps (not shown) can be present in both EGRloops to reduce EGR temperature.

This system allows partially free EGR, minimal bulk flow pumping losses(potential to eliminate intake throttle), EGR during transient-NOxcontrol, more exhaust flow through turbine during normal operation-lessturbo lag, all exhaust flows through the turbine during transient-betterresponse, and low filling/purging volume (quick response). However,there may be durability issues. Further, this system may require pumpingEGR (high pressure, low mass flow), added cost, longer EGR plumbing,coordinated control between the loops, and slightly higher flow throughDPF (increased back pressure).

EGR System #5: 5 to 4 & 7 to 1 (see FIG. 16 d)

This system includes two EGR loops. One EGR loop connects the exhaustmanifold to the intake manifold, like those used currently. The othertakes exhaust gases from a point after the DPF and pumps it to a pointbefore the compressor using either an air pump or a venturi. When thevehicle is not accelerating (no NOx spike predicted), the first EGR loopis used as much as is needed, and the second loop is used to supplementthe EGR flow if required, thus avoiding the use of the intake throttle.When the vehicle is trying to accelerate (or a NOx spike is predicted),the first EGR valve is shut to maximize the exhaust flow through theturbine, thus improving transient response of the engine. The secondloop is used to supply a base amount of EGR to the engine to control NOxemissions. Optional Coolers (Y) cab be present in both EGR loops toreduce EGR temperature. Further, optional pumps (Z) could also be used.

This system allows partially free EGR, minimal bulk flow pumping losses(possibility to eliminate intake throttle), EGR during transient-NOxcontrol, more exhaust flows through turbine during normal operation-lessturbo lag, all exhaust flows through the turbine during transient-betterresponse, low filling/purging volume for most EGR flow (quick response),some of EGR, most during transient operation, will be well mixed, andlikely to use a venturi instead of an air pump (low pressure, low massflow). However, in additional to durability, there may be added cost dueto the dual loops, longer EGR plumbing, coordinated control, andslightly higher flow through DPF (increased back pressure).

The inventors here have conducted tests both on an engine dynamometerand in a vehicle and have found a benefit to using this loop to NOx(˜10-12% reduction) without sacrificing engine performance or fueleconomy. In fact, there may even be a fuel economy benefit to using thissystem over the convention system, system #1), depending on the engineand system configuration.

EGR System #6: 7 to 2 (see FIG. 16 e)

This system has essentially the same benefits and disadvantages as EGRsystem #3, with only two exceptions. First, the pressure just after thecompressor is slightly higher than in the intake manifold, thereforeflowing EGR in this loop would be more difficult. Second, using theintercooler to cool the EGR with the intake air would introduce apotential cost save since it would eliminate the need for an EGR cooler.An optional pump Zc, optional intercooler Yc, are shown routing exhaustgas via line 202 c upstream of the device 19 b. A vacuum regulator 224 cis also shown providing an EGR pressure signal (EGRPc), along with anEGR valve 200 c. Note that System #6 can be considered as a possiblevariation of the low to high pressure EGR loop configuration of system#2.

EGR System #7: 5 to 4 & 7 to 2 (see FIG. 16 f)

This system has essentially the same benefits and disadvantages as EGRsystem #5, with only two exceptions. First, the pressure just after thecompressor is slightly higher than in the intake manifold, thereforeflowing EGR in this loop would be more difficult. Second, using theintercooler to cool the EGR with the intake air would introduce apotential cost save since it would eliminate the need for an EGR cooler.Note that System #7 can be considered as a possible variation of system#4.

Finally, note that systems 4, 5, and 7 are especially suited for usewith the strategy described above herein. Thus, according to the presentinvention, it is possible to use a dual loop EGR system to reduce NOx.In one example, the two EGR loops are each adjusted via valves tocontrol the amount of EGR based on engine operating conditions. In someconditions, both EGR loops are used, and in other conditions, only oneof the loops it utilized. Further still, in other example, neither loopis used.

In one specific example described above, at least two operating modesare utilized. A first mode is used where two EGR loops are utilized(both a high pressure loop and a low pressure loop). In this case, thelop pressure loop is used to put in a low level of EGR while the highpressure loops is used to control EGR amount in total (or airflow). Asecond mode uses only the low pressure EGR loop to maximize the massflow through the turbine.

In this way, it is possible to minimize NOx emissions, even duringtip-in NOx spikes.

This concludes the detailed description. As noted above herein, thereare various alterations that can be made to the present invention.

1-4. (cancelled).
 5. A system for an engine having an intake and exhaustmanifold, the engine having a compression device coupled with a firstportion coupled to the intake and a second portion coupled to theexhaust manifold of the engine, the system comprising: a first exhaustgas recirculation system having a first end coupled to the exhaustmanifold upstream of the second portion of the compression device and asecond end coupled to the intake manifold downstream of the firstportion compression device, said first system also having a first valvethat adjusts a first flow amount from the exhaust manifold to the intakemanifold; and a second exhaust gas recirculation system having a firstend coupled downstream of the second portion of the compression deviceand a second end coupled upstream of the first portion of thecompression device, said first system also having a second valve thatadjusts a second flow amount from the exhaust to the intake.
 6. Thesystem recited in claim 5 further comprising a controller electricallycoupled to said first and second valve.
 7. The system recited in claim 6wherein said controller determines engine operating conditions, andactuates both said first and second valve during a first mode, and onlysaid first valve during a second mode based on said determined operatingconditions. 8-24. (cancelled)